Radio frequency (RF) and microwave switches are used to switch between different high frequency transmission paths when the different transmission paths are isolated from each other. Conventional RF and microwave switches are implemented in numerous types of systems, such as test systems, RF front-ends, and the like.
Generally, there are two groups of RF and microwave switches: electromechanical switch and solid state switches. Electromechanical switches use electromagnetic induction to switch among mechanical contacts. Solid state switches, however, do not have moving parts. Due to certain favorable parameters of solid state switches, such as high switching speed, short settling time and long operating life, they typically are used in mobile phone or other RF front-end modules (FEMs) for time division duplex (TDD) applications, such as TDD switched power amplifier duplexer (S-PAD) modules. Since solid state switches are based on semiconductor technologies, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), they inherently have capacitive behavior. This inherently capacitive behavior will act as an impedance transformer which affects the matching of the circuits connected at the switch input and switch output. Even if the circuits connected at switch input and switch output are matched to each other (e.g., both 50 Ohms) by absence of the solid state switch, the solid state switch will lead to a mismatch when added between these two circuits due to its inherently capacitive behavior. Therefore, the capacitive impedance of a solid state switch must be compensated for or transformed to a design impedance (e.g., 50 Ohms) by a matching circuit.
FIG. 1A is a simplified block diagram of a conventional compensation/matching circuit. Conventional compensation/matching circuits typically incorporate a shunt inductor. For example, referring to FIG. 1A, matching circuit 100 includes a single shunt inductor 110 to compensate for the capacitive behavior of the solid state switch 120 (or other comparable inherently capacitive device) to provide matching between the solid state switch 120 (or other comparable inherently capacitive device) and the filter 130 (or other electronic circuit, such as an antenna, a power amplifier, or a common receive (Rx) PAD), for example. The compensation network (e.g., the shunt inductor 110) may alternatively be placed on the opposite side of the solid state switch 120, instead of between switch 120 and device 130, to achieve the same result. Such compensation/matching, however, results in limited bandwidth, which is insufficient for next generation S-PAD modules. Broadband solutions for compensation/matching circuits are needed in various scenarios, such as covering an increased number of mobile bands, using multiple TDD bands simultaneously, using the same antenna for multiple frequency bands, covering multiple frequency bands by a single broadband multiband power amplifier, and combining different receive paths at a common Rx pad.
FIG. 1B is a simplified block diagram of the conventional matching circuit 100 from the perspective of the solid state switch 120. More particularly, the solid state switch 120 has an input port 121 and an output port 122. The design impedance 161′ (related to reference ground) at the point of connection (port 161) of the input port 121 to another component (e.g., at reference plane 131) is typically about 50 ohms, and the design impedance 162′ (related to ground) at the point of connection of the output port 122 to another component (e.g., at reference plane 132) is also typically about 50 ohms. Notably, indication of the impedances 161′ and 162′ would not be present when the corresponding input port 121 and output port 122 of the solid state switch 120 are shown connected to other components, such as the filter 130 in FIG. 1A.
The shunt inductor 110 of the conventional matching circuit 100 compensates for the capacitive part of the solid state switch 120 by transforming impedance. For example, FIG. 2A is a Smith chart showing return loss of the solid state switch 120 in an ON state and matching with conventional shunt inductor 110. The shunt inductor 110 is dimensioned so that the entire frequency band (e.g., from 1.8 GHz to 2.7 GHz) is matched as well as possible. Therefore the shunt inductor 110 is dimensioned so that the impedance m1 at the left edge of the frequency band to be matched (e.g., about 1.8 GHz), and the impedance m2 at the right edge of the frequency band to be matched (e.g., about 2.7 GHz) are approximately complex conjugated with respect to one another. Curve 223 between the impedances m1 and m2 represents impedances of the complete frequency band to be matched, where the impedances m1 and m2 are the edges of this frequency band. In this example, for instance, the impedance m1 is indicated as complex number Z0*(0.720+j0.409), and the impedance m2 is indicated as complex number Z0*(0.722−j0.408), where Z0 is the reference impedance used for the Smith chart. The resulting matching resonance frequency is somewhere in the middle of the frequency band to be matched, which in this example is about 2.2 GHz (as indicated by negative peak 222 in FIG. 2B and positive peak 224 in FIG. 2C, discussed below). The impedance m0 in FIG. 2A is the resulting impedance at this matching resonance frequency of the solid state switch 120 by matching with conventional shunt inductor 110. As is evident by the Smith chart, the impedances on the return loss curve 223 in FIG. 2A between the impedance m1 at the left edge of the frequency band to be matched and the impedance m0 approximately at the middle of the frequency band to be matched are inductive, while the impedances on the return loss curve 223 between the impedance m0 and the impedance m2 at the right edge of the frequency band to be matched are capacitive, although the impedances between the impedance m0 and the impedance m2 are less capacitive than the inherently capacitive solid state switch 120 without the shunt inductor 110. The matching resonance frequency is the frequency point at which matching is best (that is, nearly ideal), and therefore lies approximately within the middle of the frequency band to be matched.
FIG. 2B is a graph showing return loss (S-parameter S11) of the shunt inductor matched solid state switch 120 in ON state in decibels (dB) at reference plane 131, corresponding to the design impedance 161, as a function of frequency in GHz, and FIG. 2C is a graph showing insertion loss (S-parameter S21) of the shunt inductor matched solid state switch 120 in the ON state in dB as a function of frequency in GHz. However, the achieved impedance matching has limited bandwidth, which is not acceptable for next generation TDD (Time Division Duplex) multiband applications. For example, assuming FIGS. 2B and 2C depict return and insertion loss for impedance matching frequency band 38 (2570 MHz-2620 MHz) and frequency band 39 (1880 MHz-1920 MHz) in a composite broadband frequency range using the one shunt inductor 110, the return loss would be only about 10 dB at the left and right edges of the frequency band to be matched, which is not sufficient.